1. Field of the Invention
The present invention relates to balanced acoustic wave filter devices using surface acoustic waves or boundary acoustic waves, and more particularly, to a balanced acoustic wave filter device including first and second longitudinally coupled resonator-type acoustic wave filter sections and having a balanced-to-unbalanced conversion function.
2. Description of the Related Art
In mobile communication apparatuses, surface acoustic wave filters are often connected as band-pass filters between antennas and differential amplifiers. In this case, unbalanced signals are input or output to an antenna. In contrast, balanced signals are input or output to a differential amplifier. Thus, a component having an unbalanced-to-balanced conversion function, that is, a balun, must be inserted between the antenna and the differential amplifier. If a surface acoustic wave filter having a balanced-to-unbalanced conversion function is used as the above-mentioned band-pass filter, the balun may be omitted. Thus, various balanced surface acoustic wave filters having the balanced-to-unbalanced conversion function have been proposed.
Japanese Unexamined Patent Application Publication No. 2003-78387 (Patent Document 1), which will be described below, discloses a balanced surface acoustic wave filter device whose electrode structure is shown by a schematic plan view illustrated in FIG. 14. As shown in FIG. 14, in a balanced surface acoustic wave filter device 501, electrodes are provided on a piezoelectric substrate. That is, the electrodes are arranged to define a first longitudinally coupled resonator-type surface acoustic wave filter section 502 and a second longitudinally coupled resonator-type surface acoustic wave filter section 503.
The first surface acoustic wave filter section 502 includes first to third IDTs 511 to 513 disposed along a surface-acoustic-wave propagation direction. The second surface acoustic wave filter section 503 also includes fourth to sixth IDTs 514 to 516 disposed along the surface-acoustic-wave propagation direction. Reflectors 517a and 517b are provided on both sides in the surface-acoustic-wave propagation direction of an area in which the IDTs 511 to 513 are disposed. Similarly, reflectors 518a and 518b are provided on both sides in the surface-acoustic-wave propagation direction of an area in which the IDTs 514 to 516 are disposed.
That is, the surface acoustic wave filter sections 502 and 503 are longitudinally coupled resonator-type surface acoustic wave filter sections each defined by three IDTs having the above-mentioned electrode structure.
In addition, first ends of the IDTs 512 and 515, which are disposed at the centers of the first and second surface acoustic wave filter sections 502 and 503, respectively, are connected to an unbalanced terminal 504. Second ends of the IDTs 512 and 515 are connected to a ground potential.
First ends of the IDTs 511 and 513 are commonly connected to a first balanced terminal 505. Second ends of the IDTs 511 and 513 are connected to the ground potential. In contrast, first ends of the IDTs 514 and 516, which are disposed on both sides in the second surface acoustic wave filter section 503, are commonly connected to a second balanced terminal 506. Second ends of the IDTs 514 and 516 are connected to the ground potential.
In the surface acoustic wave filter device 501, the phase of the IDT 512 is opposite to the phase of the IDT 515. Thus, the phase of a signal flowing to the first balanced terminal 505 differs from the phase of a signal flowing to the second balanced terminal 506 by 180 degrees.
Since the surface acoustic wave filter device 501 is configured as described above, the surface acoustic wave filter device 501 has a balanced-to-unbalanced conversion function.
However, in the surface acoustic wave filter device 501, satisfactory insertion loss and VSWR in a pass band cannot be achieved. That is, in the surface acoustic wave filter device 501, since the polarity of the IDT 512 is opposite to the polarity of the IDT 515, the phases of signals extracted from the first and second balanced terminals 505 and 506 differ from each other by 180 degrees.
Thus, in a gap in which IDTs are adjacent to each other, the polarities of a pair of adjacent electrode fingers differ between the first surface acoustic wave filter section 502 and the second surface acoustic wave filter section 503. For example, in the first surface acoustic wave filter section 502, each of the electrode fingers that are adjacent to each other across a gap between the IDTs 511 and 512 is connected to a corresponding signal terminal, and each of the electrode fingers that are adjacent to each other across a gap between the IDTs 512 and 513 is connected to a corresponding signal terminal. Thus, surface acoustic waves are not excited very strongly between the IDTs 511 and 512 and between the IDTs 512 and 513.
In contrast, one of the electrode fingers that face each other through a gap between the IDTs 514 and 515 in the second surface acoustic wave filter section 503 is connected to a signal terminal, and the other one of the electrode fingers is connected to the ground potential. Similarly, one of the electrode fingers that are adjacent to each other between the IDTs 515 and 516 is connected to the ground potential, and the other one of the electrode fingers is connected to a signal terminal. Thus, in the gap between the IDTs 514 and 515 and the gap between the IDTs 515 and 516, surface acoustic waves are excited relatively strongly. Thus, a bandwidth tends to be increased.
Therefore, the excitation states of surface acoustic waves in gaps in the first longitudinally coupled resonator-type surface acoustic wave filter section 502 and the second longitudinally coupled resonator-type surface acoustic wave filter section 503 differ from each other. This causes a difference in bandwidths and deteriorates the VSWR and the insertion loss in the pass band.